cgroup: prevent spurious transition into non-frozen state

If freezing of a cgroup races with waking of a task from
the frozen state (like waiting in vfork() or in do_signal_stop()),
a spurious transition of the cgroup state can happen.

The task enters cgroup_leave_frozen(true), the cgroup->nr_frozen_tasks
counter decrements, and the cgroup is switched to the unfrozen state.

To prevent it, let's reserve cgroup_leave_frozen(true) for
terminating processes and use cgroup_leave_frozen(false) otherwise.

To avoid busy-looping in the signal handling loop waiting
for JOBCTL_TRAP_FREEZE set from the cgroup freezing path,
let's do it explicitly in cgroup_leave_frozen(), if the task
is going to stay frozen.
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/freezer.c

@@ -139,19 +139,13 @@ void cgroup_leave_frozen(bool always_leave)
 		cgroup_update_frozen(cgrp);
 		WARN_ON_ONCE(!current->frozen);
 		current->frozen = false;
+	} else if (!(current->jobctl & JOBCTL_TRAP_FREEZE)) {
+		spin_lock(&current->sighand->siglock);
+		current->jobctl |= JOBCTL_TRAP_FREEZE;
+		set_thread_flag(TIF_SIGPENDING);
+		spin_unlock(&current->sighand->siglock);
 	}
 	spin_unlock_irq(&css_set_lock);
-
-	if (unlikely(current->frozen)) {
-		/*
-		 * If the task remained in the frozen state,
-		 * make sure it won't reach userspace without
-		 * entering the signal handling loop.
-		 */
-		spin_lock_irq(&current->sighand->siglock);
-		recalc_sigpending();
-		spin_unlock_irq(&current->sighand->siglock);
-	}
 }
 
 /*

kernel/signal.c

@@ -2514,7 +2514,7 @@ bool get_signal(struct ksignal *ksig)
 		 */
 		if (unlikely(cgroup_task_frozen(current))) {
 			spin_unlock_irq(&sighand->siglock);
-			cgroup_leave_frozen(true);
+			cgroup_leave_frozen(false);
 			goto relock;
 		}